From: Daniel Schierbeck <daniel.schierbeck@...>
Date: 2006-04-02T23:23:44+09:00
Subject: Re: Add a Module#attr_validator method

Trans wrote:
> Thanks Daniel, I'll give this some consideration.
> 
> A more interesting alternative that could soon come to pass is an
> extension to Facets' annotation system. Right now one can easily do:
> 
>   require 'facet/annotation'
> 
>   class Foo
>     attr_accessor :x
>     ann :x, :validation => proc { |x| ... }
>   end
> 
> But this is nothing more than a note. For it to actually have any
> effect one would have to manually utilize the note. With then next
> version of annotation.rb I plan to add annotation hooks. By defining a
> hook you can automatically have it do what you would like when a
> particular annotation is define. For instance:
> 
>   Annotation.annotation_action( :validation ) { |symbol, validator|
>     wrap_method( "#{symbol}=") { |target, x|
>       unless validator.call( x )
>         raise ArgumentError, "Invalid value provided for #{symbol}"
>       else
>         target.call(x)
>       end
>     }
>   }
> 
> Now when you make the above annotation it will automatically generate
> the validating setter as you describe.
> 
> The only downside to this kind of thing at the moment is the lack of an
> adequate AOP foundation built into Ruby.
> 
> But all that is down the road yet. In the mean time I will likely add
> your method. Thanks.
> 
> T.
> 
> P.S. Using this list is fine, especially if you're interested in wider
> input from the Ruby community. You can also email the Nitro list
> --Facets is a key component of Nitro/Og. And of course you can always
> email me directly.

Thanks for considering my idea. Personally, I like to think of a 
validator as a setter/writer that adds value validation - annotations 
just don't fit in my perception of that process, though what you 
describe would be useful, too.

Cheers,
Daniel Schierbeck